There are circumstances involving electrochemical reactions when it is necessary to provide a particular environment, e.g. alkaline conditions, for a reaction of interest to take place, or in which a particular reagent, e.g. hydrogen ions, is required to participate in a reaction of interest. Hitherto this has generally been achieved by supplying an appropriate reagent to the reaction mixture, e.g. by depositing a releasable layer of reagent within an electrochemical cell or by supplying reagent in solution to the cell. Practical difficulties can arise in certain situations. For example it can be difficult to deposit more than one layer of reagent in such a way that all layers are efficiently released into solution. Further, supplying a measured or known quantity of reagent in solution requires appropriately accurate measurement. The present invention provides an alternative approach.